Studies with intact tissues to determine the role of a Na+-Ca2+ exchange system in the cell membrane of vascular smooth muscle have resulted in conflicting reports. However, a specific Na+- Ca2+ exchange system has been consistently observed in isolated cell membrane vesicles as well as in isolated cells. The goal of this project is to determine the role of this Na+-Ca2+ exchange system of the cell membrane of vascular smooth muscle in the regulation of cytosolic Ca2+ concentration and cell contraction. Studies are designed in this project (1) to elucidate the kinetic characteristics, the stoichiometry and the regulation of the Na+- Ca2+ exchange system of sarcolemmal vesicles isolated from dog mesenteric artery and (2) to determine the changes in cytosolic Ca2+ and contraction of isolated single cells of vascular smooth muscle under conditions when the specific activity or the equilibrium of the Na+-Ca2+ exchange system of cell membrane is altered by changing intracellular and extracellular Na+ concentration. The kinetic characteristics, the stoichiometry and the regulation of the Na+-Ca2+ exchange system will be studied in isolated sarcolemmal vesicles by measuring uptake or release of Ca2+ in exchange for Na+ by radiochemical assay with Millipore filtration. The cytosolic Ca2+ concentration will be measured with the calcium-sensitive dye fura-2 and digital imaging microscopy, and the contractile state will be assessed by utilizing an interactive computer graphics system to measure cell length. Since Na+ concentration has been shown to increase in vascular smooth muscle cells in several forms of hypertension and since cytosolic Ca2+ concentration plays an essential role in cell contraction, the proposed study will provide answers to the question of whether or not the Na+-Ca2+ exchange system of the cell membrane can play any role in the development or in the maintenance of hypertension by regulating cytosolic Ca2+ concentration. Elucidation of the role of this process is likely to be of considerable importance not only towards understanding vascular smooth muscle cell function in normal and in pathologic conditions but also towards rational development of therapeutic interventions.